Prior to the subject invention, a specific intergrated circuit device used in customer premises telephone applications had been configured as shown in FIG. 1. The device as shown therein includes both a VOICE circuit coupled across the inpout terminals (TIP and RING leads) and a low-level "DIALER" circuit. The DIALER circuit might include, for example, UDK or PULSE DIAL functons and repertoire dialing. Because the DIALER circuit imposes only modest current demands, on the order of 0.85 ma, its power may be derived from a large-valued capacitor, CL, charged through a series-connected resistor R1 and Schottky diode, CR1. The VOICE circuit, on the other hand, is required not only to supply a specified output power level but also so sink current in an amount equal to at least 8 ma in order to prevent the central office from "dropping out". In order to accomplish this, the prior art implementation had utilized a PNP bypass transistor (not shown in FIG. 1) to carry 8 ma from the TIP to the RING terminals of the subscriber line.
A salient component of the subject invention is a recognition that some part of that 8 ma may be used to charge C1 and thereby supply the current necessary for operation of the DIALER circuit. Two concerns are brought to bear on the design of a circuit intended to accomplish this result. First, because the impedance across the input terminals must be maintained at a specified high level, it is important that the bypass transistor not be driven into saturation. Second, the current driven into storage capacitor C1 must not be allowed to result in a voltage greater than the DIALER circuit can tolerate. A particular DIALER circuit with which the invention was used required that the voltage across C1 not exceed approximately 6 volts.